1. Field of the Invention
The present invention relates to a method for controlling a panning/tilting motor of a monitoring camera. More particularly, the present invention relates to a method for controlling a panning/tilting motor in a closed loop using a brushless DC (BLDC) motor without employing a position detection device such as an encoder.
2. Description of the Related Art
A speed dome camera which is an example of a closed-circuit TV (hereinafter referred to as “CCTV”) monitoring system is a camera that detects motion in a monitoring area and moves in horizontal and vertical directions through a panning/tilting function. The camera is mounted in a hemispheric or spherical housing.
FIG. 1 is a schematic view of a conventional control system for a monitoring camera to control the panning/tilting function, in which the control system includes a panning/tilting monitoring camera 12, a control panel 14 (e.g., a controller) for transmitting a panning/tilting control command, and an image output 16 for displaying an image output from the camera.
The panning function is to rotate the camera in a 360 degree panning direction on a horizontal plane by a panning motor, and the tilting function is to rotate the camera in a 90 degree tilting direction on a vertical plane by a tilting motor. The monitoring area can be expanded by using the panning/tilting function.
The panning motor and the tilting motor employ a stepping motor which drives at variable speed. The panning/tilting direction and speed of the motor may be controlled according to a step (i.e., drive pulse) output from the control panel 14 of the speed dome camera.
Such a speed dome camera includes a panning sensor for initializing the position of the panning motor and a tilting sensor for initializing the position of the tilting motor. When the power is turned on, the panning motor moves from an actual position to a panning sensor position, and the tilting motor moves from an actual position to a tilting sensor position. The counter of the drive pulse is initialized after the power is turned on to allow the positions of the panning motor and tilting motor to be determined through the counting of the output drive pulse. Hence, the speed dome camera can be adaptively driven.
As described above, the position control unit of the panning/tilting camera includes the panning motor for allowing the camera to move in the panning direction, the tilting motor for allowing the camera to move in the tilting direction, and a control circuit for controlling the panning motor and the tilting motor. Position control is classified into open loop control and closed loop control according to the control mode of the motor. FIG. 2A shows a conventional open loop control system. FIG. 2B shows a conventional closed loop control system.
The open loop control system of FIG. 2A includes a stepping motor 26 having no feedback signal, a microcomputer 22 for controlling moving speed and moving direction of the motor, and a driver 24 for receiving an output signal from the microcomputer 22 and generating a motor drive current to drive the stepping motor 26 and having a pulse generator and a driver integrated circuit(IC). Such an open loop control system controls the speed of the motor according to frequency variation of a clock signal CLK provided to the driver 24 from the microcomputer 22, and controls the moving direction of the motor through the control of the high/low state of a direction signal DIRECTION provided to the driver 24 from the microcomputer 22.
Specifically, the microcomputer 22 varies the clock signal CLK and the direction signal DIRECTION to control the motor. Whenever the driver 24 outputs the pulse to the motor, the microcomputer 22 counts the number of the pulses to determine the position of the motor.
A major feature of the stepping motor is that the motor rotates corresponding to the pulse power. Since an angle of rotation is varied in proportion to the number of the pulses, and the rotation speed is varied in proportion to an input frequency, the stepping motor can control the operation of the motor without performing the feedback.
The closed loop control system of FIG. 2B includes a DC/BLDC/AC motor 36 with an encoder, a microcomputer 32 for creating a pulse width modulated (PWM) output to drive the motor, and a driver 34 for applying a motor drive current to the DC/BLDC/AC motor 36 according to an output signal of the microcomputer 32.
With the closed loop control system, the motor 36 is provided with the encoder to control the position of the motor and thereby improve position accuracy. More specifically, three encoder output signals corresponding to an amount of rotation are fed back from the encoder to the microcomputer 32. The microcomputer 32 outputs six PWM signals to the driver 34, and the driver 34 passes six PWM signals through an inverter circuit (not shown) to apply 3-phase PWM voltage to the DC/BLDC/AC motor 36. A sine-wave current having a phase difference of 120 degrees is applied to the DC/BLDC/AC motor 36 by the 3-phase PWM voltage to rotate the motor. In addition, whenever the motor 36 rotates, the output of the encoder is fed back to the microcomputer 32. The precision of the motor position control is determined by resolution of the encoder.
Alternatively, the conventional DC motor may employ a discrete closed loop control system using an output of a hall sensor only to control the position of the motor, which is not shown.
Terminals of a microcomputer are connected to six inputs of an inverter circuit, and the inverter circuit outputs three signals (i.e., A-phase, B-phase, and C-phase signals) to the motor through a combination of six high/low-state signals to rotate the motor. When the motor rotates, an output voltage of the hall sensor is fed back to the microcomputer to determine, the position of the motor.
The conventional control systems described above have the following drawbacks.
The drawback of the open loop control system of FIG. 2A using the stepping motor shall now be described. First, since there is no feedback signal corresponding to the position of the motor, position information is wrong when the motor is out of step. Second, a torque property is lowered in comparison to the DC motor. Specifically, since the monitoring camera has to continuously operate for the purpose of the product and is mainly installed outdoors, the motor is prone to stepping out due to vibration, wind or the like. Hence, the open loop control system using the stepping motor of FIG. 2A in the monitoring camera has the cost advantage, but lacks a desired level of position accuracy.
The closed loop control system of FIG. 2B using the DC/AD/BLDC motor has an advantage in that the accurate position of the motor can be determined through the feedback signal; however, it is limited to the utilization of an additional encoder to control the position. In addition, since the accuracy of the position control is influenced by resolution of the encoder, an expensive encoder of a high resolution is needed for more accurate position control.
Also, in the above-described discrete closed loop control system that controls the position of the motor based on the output of the hall sensor of the DC/BLDC motor, since the position accuracy becomes an electronic angle of 60 degrees, it is not employed in a system that requires an accurate position control such as a monitoring camera. This is because an actual rotational angle of the motor is determined by multiplying the electronic angle by the number of poles of the motor.